The field of the invention pertains to fiber optic sensors that are sensitive to pressure or stress in a manner that causes a beam of light to be reflectively modulated in response to changes in pressure or stress on the sensor. Determination of cylinder pressure and the sensors that report the pressure are currently sought by automotive manufacturers for engine control and monitoring applications. The desired product must be inexpensive and meet stringent quality, durability and reliability requirements. Additionally, the sensor package must be robust and easily routed for placement into an engine or engine head.
Ideally, the most desirable manner in which to introduce a pressure sensor into an engine is through an existing cylinder opening. By integrating a sensor with an existing device mounted into a cylinder hole, sensors can be introduced inexpensively into new engines as well as being provided for retrofit to engines already in use.
A spark plug represents a desirable device for integration with a pressure sensor as the spark plug partially enters the engine and thusly provides a preexisting opening into the engine. While special pressure measurement spark plugs have been used in engine research with expensive sensors, such research devices are not suitable for use in production engines. Fiber optic sensors are the best candidates for integration with a spark plug.
The small size of the sensor, the resistance to extreme temperatures, non-metallic construction and immunity to electromagnetic interference are the main advantages of fiber optic sensors over more conventional devices such as piezoelectric or piezoresistive devices.